In this type of technical field, a next-generation mobile communication system of the so-called third-generation system has been studied by 3GPP (3RD Generation Partnership Project) which is a standards body of the W-CDMA (Wideband Code Division Multiple Access). Especially, as a next-generation system of the W-CDMA (Wideband Code Division Multiple Access) system, the HSUPA (High Speed Uplink Packet Access) system, the HSDPA (High Speed Downlink Packet Access) system and the like, a Long term Evolution (LTE) system has been studied at high speed. In the LTE system, as a radio access system, an OFDM (Orthogonal Frequency Division Multiplexing) scheme and an SC-FDMA (Single-Carrier Frequency Division Multiple Access) scheme have been studied to be applied to the downlink communications system and the uplink communications system, respectively (see, for example, Non-Patent Document 1).
The OFDM scheme is a multi-carrier transmission scheme in which a frequency band is divided into plural narrower frequency bands (sub-carriers), and data to be transmitted are mapped onto the sub-carriers. By closely and orthogonally arranging the sub-carriers along the frequency axis, the achievement of faster transmission and further improvement of the efficiency of using the frequency are expected.
The SC-FDMA scheme is a single carrier transmission scheme in which a frequency band is divided for each user equipment (hereinafter may be referred to as a user equipment (UE) terminal) in a manner such that different frequencies can be separately used (allocated) among plural terminals (user equipment (UE) terminals). as a result, interference between the terminals may be easily and effectively reduced. Further, preferably, in the SC-FDMA scheme, a range of transmission power fluctuation may be made smaller; therefore, lower energy consumption in terminals may be achieved, and a wider coverage area may also be obtained.
In both uplink and downlink of the LTE system, communications are performed by allocating one or more resource blocks (RBs) or resource units (RUs). The resource blocks are shared among the plural user equipment (UE) terminals. In the LTE system, the base station apparatus determines which resource blocks are allocated to which user equipment (UE) terminals among the plural user equipment (UE) terminals for each sub-frame having a duration of 1 ms. The sub-frame may also be called a Transmission Time Interval (TTI). The determination of the allocation of radio resources is called scheduling. In downlink communication, the base station apparatus transmits a shared channel using one or more resource blocks to the user equipment (UE) terminal selected in the scheduling. This shared channel may be called a Physical Downlink Shared CHannel (PDSCH). On the other hand, in uplink communication, the user equipment (UE) terminal selected in the scheduling transmits a shared channel using one or more resource blocks to the base station apparatus. This shared channel may be called a Physical Uplink Shared CHannel (PUSCH).
Further, in the communication system using the shared channels, it is required to perform Signaling to report which shared channels are to be allocated to which user equipment (UE) terminals for each sub-frame. To perform the Signaling, a control channel is generally used. In the LTE system, the control channel may be called a Physical Downlink Control CHannel (PDCCH) or a Downlink L1/L2 Control Channel (DL−L1/L2 Control Channel). A downlink control signal may include not only this PDCCH but also a Physical Control Format Indicator CHannel (PCFICH) and a Physical Hybrid Indicator CHannel (PHICH) and the like.
The PDCCH may include, for example, the following information items (see, for example, Non-Patent Document 2).
Downlink Scheduling Information;
Uplink Scheduling Grant;
Overload Indicator; and
Transmission Power Control Command Bit
The Downlink Scheduling Information includes, for example, information of the downlink shared channel, and specifically, allocation information of downlink resource blocks, identification information of user equipment (UE) terminal (UE-ID), the number of streams, information of Pre-coding vector, data size, modulation scheme, information of HARQ (Hybrid Automatic Repeat ReQuest) and the like.
On the other hand, the Uplink Scheduling Grant includes, for example, information of the uplink shared channel, and specifically, allocation information of uplink resource blocks, identification information of user equipment (UE) terminal (UE-ID), data size, modulation scheme, information of uplink transmission power, Demodulation Reference Signal in uplink MIMO (Multiple Input Multiple Output) and the like.
The PCFICH transmits a format of PDCCH. More specifically, the number of OFDM symbols used for the PDCCH is transmitted using the PCFICH. In the LTE system, the number of OFDM symbols used for the PDCCH is 1, 2, or 3, and the OFDM symbol(s) within a subframe are sequentially mapped from the first OFDM symbol of the subframe.
The PHICH includes Acknowledgement/Non-Acknowledgement information (ACK/NACK) indicating whether the PUSCH transmitted in uplink is required to be retransmitted.
As far as the definition of the terms is concerned, the PDCCH, the PCFICH, and the PHICH may be defined as equivalent channels independent from each other, or, for example, may be defined in a manner such that the PDCCH includes the PCFICH and the PHICH.
In uplink, the PUSCH transmits user data (normal data signal) and accompanying control information. Further, besides the PUSCH, a Physical Uplink Control CHannel (PUCCH) transmits downlink CQI (Channel Quality Indicator), the Acknowledgement/Non-Acknowledgement information (ACK/NACK) of the PDSCH and the like. The CQI is used as in, for example, the scheduling process, AMCS (Adaptive Modulation and Coding Scheme) of the PDSCH and the like. In uplink, a Random Access CHannel (RACH) and a signal requesting for the allocation of uplink/downlink radio resources may be transmitted on an as-needed basis.
FIG. 1 schematically shows an example of mapping of a downlink signal. In this example of FIG. 1, a Reference Signal (RS) and the PHICH (Physical Hybrid ARQ Indicator Channel, or ACK/NACK) are mapped to the first OFDM symbols. For example, a sub-frame having a duration of 1 ms may have two slots, each slot having a duration of 0.5 ms. One slot may include, for example, six or seven OFDM symbols. As described above, the first one up to three OFDM symbols within one sub-frame are used for the downlink control signal (and the reference signal (RS)).
On the other hand, the PHICH expresses the ACK/NACK. Because of this feature, the PHICH may be essentially expressed by one bit. However, the PHICH (ACK/NACK) is the most fundamental information in retransmission control, and may greatly influence the system throughput. In this regard, the PHICH (ACK/NACK) largely differs from other control information items. In the example of FIG. 1, the PHICH (ACK/NACK) per single user is spread by using a Spreading Factor (SF) (for example SF=4), and the PHICHs of four users are code division multiplexed in four sub-carriers (the number (4) of the users is the same as that (4) of the sub-carriers). In this example of FIG. 1, to improve the reliability, the PHICHs of the four users are mapped to three different regions on the frequency axis. In other words, the PHICHs of the four users are simultaneously transmitted using three different frequencies.
In general signal transmission, to improve the transmission efficiency, an orthogonal modulation scheme is used. However, from the viewpoint of transmission efficiency, it may not be preferable to transmit such one-bit information as the ACK/NACK by dividing it into an In-phase component (I-ch) and a Quadrature component (Q-ch). Therefore, it is thought that the PHICH should be transmitted by using only one of the two components orthogonal to each other.
FIG. 2 schematically shows a state where the PHICHs of four users are code division multiplexed on four sub-carriers and mapped to only the I-channel (I-ch). Alternatively, the PHICHs of the four users may be mapped to only the Q channel (Q-ch).
FIG. 3 schematically shows a state where the PHICHs of eight users are code division multiplexed on four sub-carriers and mapped to the I-channel (I-ch) and the Q-channel (Q-ch). In this case, the PHICH of one user is mapped to either the I-channel (I-ch) or the Q-channel (Q-ch). From the viewpoint of increasing the number of multiplexed users, it may be preferable to multiplex the PHICHs of the users as shown in FIG. 3. By mapping the ACK/NACKs of four users to the I-channel (I-ch) and mapping the ACK/NACKs of the other four users to the Q-channel (Q-ch), the number of multiplexed users (amount of transmitted information) can be doubled (see, for example, Non-Patent Document 4).    Non-Patent Document 1: 3GPP TR 25.814 (V7.0.0), “Physical Layer Aspects for Evolved UTRA,” June 2006    Non-Patent Document 2: 3GPP R1-070103, Downlink L1/L2 Control Signaling Channel Structure: Coding    Non-Patent Document 3: 3GPP TR 36.211 (V0.2.2), “Physical Channel and Modulation”, November 2006    Non-Patent Document 4: 3GPP R1-074580 “PHICH Channel Structure”, Motorola, November 2007